She was standing in the Place de la Concorde in Paris, in 1959, when she heard a sound that devastated her.

She thought she recognized it from Auschwitz-Birkenau, the Nazi death camp where Posmysz was imprisoned from 1942 to 1945. The sound instantly sent her back to that nightmare.

"There was a huge crowd, lots of tourists, and most of them German," remembers Posmysz, 91, of the incident in Paris that led her to write the story now unfolding on stage at Lyric Opera, "The Passenger."

"I heard a voice," continues Posmysz, "and so this voice sounded identical with the voice of my overseer in Auschwitz.

"And I was just shattered. And I started to think: What should I do? Should I report her to the police immediately, as a former SS Nazi?

"Or should I go to her and ask, 'Wie geht's, Frau Aufseher,'" which translates as: "How goes it, Frau Overseer?'"

Before Posmysz could make a decision, she came to realize that the voice in fact did not belong to Anneliese Franz, the Nazi guard, but was simply that of a passing German tourist.

"But then the question remained," adds Posmysz.

The question being: How do you confront the source of your pain and sorrow? Or, more broadly, how do you deal with a searing trauma that continues to haunt you?

"When I came back (to Poland), I must have been changed," says Posmysz, speaking through a translator in the sunny living room of a downtown apartment. A few blocks away, composer Mieczyslaw Weinberg and librettist Alexander Medvedev's opera – inspired by Posmysz's radio play "Passenger in Cabin 45" and a subsequent novella version, "The Passenger" – vividly shows how Posmysz decided to grapple with her past.

Zbigniew Bzdak / Chicago Tribune

Author Zofia Posmysz talks to director David Pountney on the set of "The Passenger" at the Civic Opera House in Chicago.

Author Zofia Posmysz talks to director David Pountney on the set of "The Passenger" at the Civic Opera House in Chicago. (Zbigniew Bzdak / Chicago Tribune)

"My husband asked: 'What happened? You seem strange,'" remembers Posmysz of his reaction to her when she returned from Paris.

"And I told him about that meeting. And he told me: 'You should write about it.' And that is how I wrote my first (dramatic) piece ever."

Not that Posmysz had been silent about her past until then. The first article she wrote for the Polish publication Glos Ludu (Voice of the People), in 1945, was about Auschwitz SS men on trial in Germany. She signed the article not with her name but with the Auschwitz number that's tattooed on her left arm: 7566.

But the journey from Posmysz's Auschwitz story to the profound and unnerving opera that the great composer Dmitri Shostakovich called "a perfect masterpiece" and "a hymn to humanity" has been long and arduous. Weinberg – its Jewish, Polish-born composer – had fled the Nazis from Warsaw in 1939, spent most of the rest of his life in the Soviet Union and died in Moscow in 1996, at 76, never having seen "The Passenger" staged. Librettist Medvedev died a few days after the work's astonishingly delayed premiere at the Bregenz Festival in Austria in 2010, 42 years after it was completed.

Only Posmysz (pronounced POS-mish) is still here to tell the story of how her ordeals became one of the rare works of art to effectively conjure a bit of the horrors of the Holocaust.

When the Germans invaded Poland in 1939, "all higher education was banned," says Posmysz, a Roman Catholic who was 16 at the time. This meant that the economics high school she was attending was shuttered and that she, like her peers, was ordered to report to the Nazi bureaucracy.

"If you didn't have a certificate that you actually went to the Arbeit Amt (Work Office), you would be arrested immediately. Or, worse, sent to Germany as forced labor," says Posmysz.

She was given a job as a waitress in the dining facility of the Agricultural Ministry, which was a stroke of luck because it meant she had access to food at a time when it was in short supply. But she also attended secret classes organized by the Polish underground, until 1942.

"Probably someone denounced us to the Nazis, and I was arrested with three colleagues of mine who were attending the same secret class with me," she says. "And one of the colleagues had with him some illegal materials, like leaflets. And so I was accused of (dealing in) forbidden materials.

"But it didn't' really matter, because I was condemned anyway, because secret education was forbidden as well."

After six weeks of what Posmysz calls a "brutal investigation," she was sent directly to Auschwitz.

She notes that as a non-Jew who did not have to wear the Star of David that marked one for death, her life in Auschwitz was less severe than it was for Jews.

"Beyond any doubt," she says.

But it was crushing, nonetheless.

"We could see the trains coming, because the trains came right in front of the gas chamber," says Posmysz, referring to the cattle cars packed with Jews and others who soon would be immolated.

"1944 was the most horrible of all in that respect. It was the period where they were mass murdering Hungarian Jews, so the crematories were burning day and night."

One day, near dawn, Posmysz was awakened by a strange, other worldly kind of singing, she remembers. She stepped outside of the barracks and saw "bodies on the grass. I didn't know if they were asleep or dead. They were lying there. And in the middle there was a man standing, and he was singing and raising his hands up."

A Jew asked Posmysz if she understood what she was hearing.

"I said: It must be a prayer," Posmysz responded. "And she said, 'Yes, it's the Kaddish,'" a Hebraic prayer for the dead.

By morning, "there was nobody there, and there was smoke over the chimney of the crematory. So how can you live with that kind of image in your brain?"

Posmysz struggled with this question for years, suffering nightmares but carrying on with life. Her mother told her, "You have to forget it," but, of course, that was impossible.

Instead, after the incident at Place de la Concorde, Posmysz – by then working for Polish radio – wrote "Passenger in Cabin 45," a survivor's drama with a twist. For Posmysz told the story not from her own perspective but from that of her Nazi overseer, Anneliese Franz. That real-life person emerges in the opera as Liese, and while on a cruise ship with her German diplomat husband Liese encounters Marta, a survivor who had been under her charge in Auschwitz.

In the opera, Liese is rattled by this coincidence, and when her husband presses her about her discontent – just as Posmysz's husband had done – Liese reveals her sordid past to her hitherto unknowing spouse.

Why did Posmysz choose the unlikely device of putting herself in the psyche of her tormentor?

The main reason, she says, was less artistic than practical.

"If I wrote exactly what I wrote in 'Passenger' as a former inmate, it wouldn't have been published," says Posmysz, referring to pervasive censorship in the Soviet Union at the time. Works of art, in other words, not only were supposed to conform to certain artistic values but were to honor Soviet sacrifice, not Polish or Jewish victims of the Holocaust.

"This protagonist of mine, so to speak, herself presents her defense case to her husband, and not on trial to the judges. She says, in her defense, everything she could say in her defense.

"And that was the only way to present certain facts, because there are facts in this story. And then I think I had good tactics, because the play – my drama – was allowed to be broadcast."

The formidable Polish director Andrzej Munk was moved to begin filming the story on location in Auschwitz, but he was killed in a car accident in 1961 before finishing his work. A truncated version of his feature was released and won awards at Cannes and Venice film festivals, but it's quite incomplete and leaves one wondering what might have been. Posmysz believes it would have been "a masterpiece," she says.

This setback inspired Posmysz to write a novella version, which eventually was translated into more than a dozen languages, a Russian version finding its way into Shostakovich's hands. He encouraged composer Weinberg to read it, an apt idea, considering that Weinberg's family had perished in the Holocaust.

Posmysz met with Weinberg in Moscow in the 1960s, at the composer's request, eager to hear his thoughts. But the conversation went in a different direction.

"He was just asking questions about the concentration camp," she recalls. "How was it in Auschwitz? What did the Germans do? How did they behave? And he wanted to know how an average day looked. It was a long conversation.

"I was a bit tired talking so long and a bit disappointed, because I wanted to listen to him. But I still have this impression even today that he was listening to me, but he was looking somewhere beyond me. And only later I understood why he behaved that way, when I learned that his family (was) exterminated, and that he must have been thinking about their end."

Though Posmysz repeatedly states that hers is not a documentary work but a piece of literature, she acknowledges that it is thick with autobiography. The Auschwitz overseer goes by her actual name, after all. A woman named Marta was Posmysz's closest friend in Auschwitz, and the operatic Marta bears attributes of both women's lives, says Posmysz. Marta's fiancé in the opera, Tadeusz, was inspired by the real-life Polish officer Tadeusz Paulone, who was killed in Auschwitz in 1943.

Ultimately, says Posmysz, she wrote the story so that "the names – Marta, Tadeusz, will never be forgotten."

But Soviet-bloc censorship very nearly succeeded in squelching the piece, librettist Medvedev struggling unsuccessfully for decades to get the work produced in Poland and elsewhere. That "The Passenger" finally is coming to the world's attention represents what Posmysz calls "a huge victory. Not mine. It's the victory of Weinberg's music. Without this music, maybe my book would be published, maybe not … but, actually, music carries the memory."

Certainly the lyric sweep and harmonic edge of Weinberg's score render Posmysz's message all the more potent. So does the ingenuity of Medvedev's libretto, which unfolds in parallel worlds: amid the glamour of the cruise ship and in depths of Auschwitz.

Posmysz rightly stresses that no work of art can capture what happened in Auschwitz or across the bleak landscape of the Holocaust, but "it can bring it closer" to us, she says. After Posmysz wrote "Passenger," "I stopped having nightmares," she says, but of course scars remain. Because of the toxic materials she was exposed to, she says, she was left unable to have children, "which made my life so much poorer."

But she has transformed her terrors into something illuminating and lasting, not only in the story of "Passenger" and other works but in the public speaking she does at Auschwitz to children who visit there from around the world.

"When you tell them how it was, they open their eyes in disbelief," says Posmysz.

"The Passenger" has the same effect, making Posmysz's work as much testimony as art, to be treasured as both.